Coaxial cables



June 9, 1959 H. BANDl-:s ETAL 2,890,263

' coAxIAL CABLES Filed Nov. 18, 1952 mvENToRs ermamm nde.:

United States Patent O 1 2,890,263 'CGAXIAL CABLES Hermann Brandes and Karl Andresen, Hannover, Germany, assignors to Hacketh-al Dralltund Kabel Werke A.G., Hannover, Germany, a corporation of Germany Application November 18, 1952, Serial No. 321,195

9 Claims. (Cl. 174-29) This invention relates to coaxial cables and more particularly, to cables designed to carry ultra high frequency currents.

Conventional coaxial cables comprise an outer conductor in tubular form, an inner conductor axially disposed therein and dielectric means for spacing the conductors relative to each other. Such dielectric means takes the form of solid discs, helical strips or the like of synthetic resin or the like.

When the outer conductor of a conventional coaxial cable is formed from smooth, thin wall tubing of copper or the like, the resultant cable has limited ilexibility and little resistance to crushing forces. It has been proposed to form the outer conductors of the cable with spaced indentations or corrugations, the conductors comprising a one piece or two piece construction. In either case, there is at least one longitudinal joint which requires special treatment to prevent the meeting joint edges from sliding over each other. Furthermore, such cable constructions require auxiliary metal tapes or strips to bind the sections together and to insure proper jointing at the joint edges. The use of such auxiliary binding means may be attended by slippage or displacement thereof and is unsatisfactory in an electrical sense because of radiation at the joints.

In addition to the relative inexibility of conventional cables, the same show little resistance to crushing which necessitates the use of auxiliary reinforcing means of various sorts. Despite the application of reinforcing means, there is no assurance that the relative spacing between the inner and outer conductors will be maintained at all times and particularly when the cable section is subjected to crushing forces, it being understood that the maintenance of such spacing at all times is extremely important if optimum electrical characteristics of the cable are to be obtained.

Accordingly, an object of this invention is to provide an improved coaxial cable construction which is distinguished by substantial flexibility, high resistance to crushing, constant .spacing between the inner and outer con ductors at all times and under all conditions which insures optimum operating results and which is particularly adapted to transmit currents in the ultra high frequency range, in an eicient and economical manner.

A further object of this invention is to provide a coaxial cable which includes an outer tubular conductor and an inner conductor located in coaxial relation thereto, the outer conductor being formed of a metal of high mechanical strength and good resistance to crushing, the tubular wall being relatively thin and formed in a manner to substantially increase the crush resistance thereof and also providing for improved exure characteristics in the completed cable, the elements of the cable being of a nature and in a relative arrangement as to substantially prevent any changes in the normal coaxial relationship of the outer and inner conductors upon lexure of the cable or upon the application of crushing forces or 'the like thereto.

Another object of this invention is to provide an improved coaxial cable wherein the outer conductor com prises an outer layer of metal of high mechanical strength and permeability, and an inner layer, thinner than the outer layer, of a metal of high electrical conductivity, the layers being joined by rolling or electrodeposition to form ICC 2 a strip which is readily converted to tubular form with a longitudinally extending, fluid impervious seam, the tubular form being transversely corrugated to increase the crush resistance and improve the liexure characteristics of the cable.

Still another object of this invention is to provide a coaxial cable wherein the outer conductor is particularly adapted to withstand high internal or external pressures in the absence of auxiliary reinforcing means, is adapted to flex in a manner 'to provide a continuous bending line whereby to maintain the axial disposition of the inner conductor upon exure of the cable as a Whole.

Still a further object of this invention is to provide a coaxial cable wherein the outer conductor is made of laminated metal layers so as to combine mechanical strength with optimum electrical characteristics, the conductor including a relatively thin layer of specially prepared copper having enhanced electrical characteristics; and an improved dielectric spacing the inner and outer conductors for further improving the overall electrical properties of the cable.

Other objects of this invention will in part be obvious and in part hereinafter pointed out.

Accordingly, the invention consists of the elements of construction, arrangement of parts and utilization of materials as will be exemplified in the embodiments herein shown and described, the scope of invention being indicated in the claims following.

In the drawing;

Fig. l is a longitudinal section through a portion of a coaxial cable embodying the invention;

Fig. 2 is a transverse sectional view thereof;

Fig. 3 ris a longitudinal section of a cable portion embodying the invention and showing a modification thereof;

Fig. 4 is a longitudinal section showing a further modiiication;

Figs. 5-7 are transverse sections taken on the lines 5-5, 6--6, 7-7, respectively in Fig. 3, the dielectric spacer being omitted;

Fig. 8 is a longitudinal section of a cable portion showing auxiliary reinforcing means;

Fig. 9 is a longitudinal section of a cable section, similar to that shown in Fig. 8 and illustrating a modification thereof.

Referring in detail to the drawing, and particularly to Fig. 1, 1l) designates a coaxial cable construction em bodying the invention. The same comprises an outer conductor of generally tubular form designated at 11; a tubular inner conductor 12 axially disposed within the outer conductor and spaced therefrom by means of a dielectric material having a low power factor, in the form of a helical strip 13 of a synthetic plastic such as polyethylene or the like.

The outer conductor 11 comprises a composite strip of metal including a relatively thin outer layer 14 of steel and a thin inner layer 15 of copper. The steel layer has high permeability and therefore tends to reduce the electric eld strengthl in the outer conductor and thereby prevents radiation of the high frequency energy. Additionally, the steel layer has high mechanical strength and resistance to crushing forces, thereby protecting the dielectric 13 and the inner conductor 12.

The composite metal strip is curved about a longitudinal axis to form a tubular member, the meeting edges being lapped and mechanically joined together to form a hermetic, fluid impervious seal as at 16. Such seam may be formed by a welding operation and the thickness of the metal at the lap joint'may be somewhat reduced to provide for substantially uniform tubular wall thickness.

The tube thus formed, is transversely corrugated by suitable means tovprovide continuous, helical corrugations having an essentially sinusoidal form in longitudinal section, such corrugations including peaks 17 and valleys 18.

It has been found that if the corrugations are so formed that the ratio of the median diameter of the corrugations of the outer conductor, indicated as Dm, to the inner diameter of the said conductor, indicated as D, is in a range ofA between `about 1.06 and 1.24, and preferably when such ratio is 1.12, then an optimum relationship between the ilexure characteristics, mechanical strength and electrical properties, will be obtained.

Using the ratio constant of 1.12, it will be found that the ratios of the inner diameter of the outer conductor D, to the outer diameter of the inner conductor 12, indicated as d, for several standard impedance ratings for cables having an effective dielectric constant e of 1.16, are as follows:

Impedance rating: Ramo 50 ohms 2.20

60 ohms 2.64

70 Ohms 3.15

In general, the equation for the characteristic impedance of a coaxial cable made in accordance with the invention and based upon the optimum configuration of the corrugations, may be expressed as follows:

wherein Z is the impedance in ohms, e is the effective dielectric constant of the dielectric used, D1 is the inner diameter of the outer conductor and d is the outer diameter of the inner conductor.

The inner conductor 12 may take the form of a thin walled copper tubing which may be filled with powdered glass 19, or other vitreous material, for the reasons hereinafter described. For smaller diameters, the inner conduc- -tor may be in solid form.

The dielectric spacer 13 may be in the form of an extruded strip of polyethylene, polystyrene, polyisocyanates or other known synthetic plastic having a low dielectric constant and of low power factor. Alternatively, the strip 13 may be formed of a plurality of thin tapes of the selected plastic laminated together to the required thickness.

It has been found that the coaxial cable will display improved electrical properties when the dielectric spacing the conductors is of a more homogeneous nature. Accordingly, the space between the conductors may be entirely filled with a porous, foamed synthetic plastic 20, of polyethylene, polystyrene, or the like, as shown in Fig. 3, or the dielectric may be in the form of a porous mass of synthetic plastic a, extruded in tubular form over the inner conductor 12, as shown in Fig. 4.

It will Ibe apparent that the corrugations of the outer conductor will not only increase the crush resistance thereof, but additionally will increase the exure properties of the cable. It has been found that such a cable can be bent over a fairly small radius without materially altering the axial spacing of the inner conductor with respect tothe outer conductor, thus maintaining the optimum electrical characteristics of the cable even when sharply exed or subjected to severe crushing forces. Furthermore, such a construction permits the use of dielectric spacers having little mechanical strength, since the bending forces arising from cable tlexure or the like, will be so small and well distributed, as to leave the dielectric unimpaired.

It is also noted that with the continuous helical corrugations in the outer conductor 11, all transverse sections taken at various points along the length of the cable will have identical total areas, as well as identical peripheral outlines. The sections taken progressively will be eccentrically relateddto each otherpand apparently revolving 4 about the axis in relation to the pitch of the corrugations, as indicated in Figs. 5-7. It has been found that with a constant, uniform cross sectional area at any point along the cable, optimum electrical operating characteristics are displayed. p

The highly conductive inner layer 15 of the outer conductor has a thickness which may be a fraction of the thickness of the outer layer 14. Such inner layer 15 may be derived from copper which has been deoxidized in the presence of zinc, the copper thus produced having been found to exhibit unusually good electrical characteristics. The copper layer may be secured to layer 14 by rolling or may be electrodeposited thereon. Other metals of high electrical conductivity, such as silver or the like may replace the copper in layer 15. When such metals are applied to the outer layer by electrodeposition, the composite material may be rolled and the inner layer polished.

With the lling 19 of powdered glass or the like, the tubing from which inner conductor 12 is formed, may be provided with a wall of uniform thickness, during the course of manufacture of the tubing. Furthermore, the filling helps to maintain the tubular shape of the inner conductor when the cable is flexed or otherwise subject to bending stresses, without impairing the flexibility of the inner conductor. Additionally, since the tubular shape of the inner conductor is maintained when subject to bending stresses, the forces arising from the flexure of the cable, as between the inner conductor and the dielectric contacting the same, are uniformly distributed, thereby avoiding undue injury to the dielectric.

While the use of continuous helical corrugations in the outer conductor have been found to be particularly desirable, from the point of view of ease of fabrication and overall electrical performance, the outer conductor may be formed with parallel corrugations, as shown in Fig. 4. This construction will have all the mechanical advantages of the preferred form of the invention and a major portion of the electrical properties thereof. It is understood that the precise configuration of the corrugations, in either case, may be suitably modified.

While the outer conductor preferably includes steel or other crush resistant metal or alloy, in cases where crushing forces may be of minimum value or altogether absent, the outer conductor, corrugated as described, may be formed of copper or other metal or alloy having the desired electrical properties and good tlexure characteristics.

In the event that conditions involving corrosion and/ or usually high longitudinal stresses are to be anticipated, cable constructions shown in Figs. 8 and 9, may be used. Thus, .as shown in Fig. 8, the outer conductor 11 is proofed against corrosion by means of an outer layer of suitable plastic material 21 known in the art, followed by a spiral wrapping of a tape 22 which may be formed of a synthetic plastic such as polystyrene, cellulose acetate or the like, the tape being covered with a cushioning layer 23 of pulp, felt or the like and finally a braided wire covering of galvanized steel wires 24 which may be protected against corrosion by a coating of elastic lacquer. The wire braid 24 is suitably anchored at the opposite ends of the cable and absorbs the tensile stresses acting on the cable. The outer covering 24 grips the cable assembly tightly and is supported by the outer conductor 11 which is able to resist substantial compression stresses.

In Fig. 9, the auxiliary reinforcing means against unusual tensile stresses takes the form of flexible, stranded steel wire 26 located within the inner conductor 12 and held in an axial position by a suitable, elastic spacing means 25 which may be of tubular shape, as shown, or may take the form of a helical strip or the like. The wire 26 is suitably anchored at the opposite ends of the cable so that it can absorb tension stresses and thereby protect the corrugated outer conductor 11 from excessive longitudinal stresses.

'This application is a continuation in part of application Ser. No. 290,994, filed April 11, 1952.

It will be apparent that there has been provided co axial cables embodying the invention wherein the several objects of the invention are achieved and which are Well adapted for conditions of practical use.

As various possible embodiments might be made of the above described invention, and as various changes might be made in the embodiments set forth, it is to be understood that Aall matter herein set forth or shown in the accompanying drawing is to be interpreted as illustrative and not in a limiting sense.

Having thus described our invention, we claim as new and desire to protect by Letters Patent:

l. A crush resistant, ilexible, high frequency coaxial cable comprising a tubuiar outer conductor, an inner conductor disposed axially within said outer conductor, and a solid dielectric member extending from said inner conductor to inner ace portions of said outer conductor for retaining said conductors in concentric relation throughout the length thereof, said dielectric member comprising a portion of the total volume of space between said conductors, the balance of said space being gas, said outer conductor being formed of metal strip curved about a longitudinal axis to tubular form and having a longitudinally extending iiuid impervious seam, said outer conductor being steel with a layer of metal of high electrical conductivity on the inner surface thereof, said outer conductor being formed with transversely extending helical corrugations, the ratio of the median diameter of said corrugations to the inner diameter thereof being between about 1.06 and about 1.24 to render said cable flexible and to increase the mechanical strength of said cable whereby to make said cable resistant to crushing forces applied thereto, said outer conductor being operative to maintain said inner conductor in axial relation thereto and to maintain optimum electrical characteristics of said cable upon ilexure of said cable over a relatively small radius or upon application of substantial crushing forces to said outer conductor.

2. A cable as in claim 1, wherein said layer of metal of high electrical conductivity is copper deoxidized with zinc.

3. A cable as in claim 1, wherein said dielectric means comprises a body of homogeneous, porous synthetic resin of low dielectric constant and low power factor.

4. A crush resistant, flexible high frequency coaxial cable comprising a tubular outer conductor, an inner conductor axially disposed within said outer conductor, an annular air space between said conductors, a dielectric member within said air space for spacing said conductors, said outer conductor comprising a transversely curved strip of metal having a longitudinally extending fluid impervious seam, said outer conductor comprising an outer layer of metal having high permeability and mechanical strength and an inner layer of metal having high electrical conductivity and relatively low mechanical strength, said outer conductor having a longitudinal section of undulating shape wherein all transverse sections taken along the longitudinal extent of said cable have the same area and peripheral outline, said dielectric member extending from said inner conductor to the valleys of said outer conductor, the ratio of the median diameter of the undulations of said outer conductor to the inner diameter thereof being between about 1.06 and about 1.24 to provide a radial depth for said undulations operative to prevent displacement of the inner conductor relative to the outer conductor and to maintain the optimum electrical properties of the cable when said cable is subjected to crushing forces or is bent over a small radius.

5. A cable as in claim 4 wherein the impedance rating of the cable may be expressed as follows:

wherein Z is the impedance in ohms, e is the dielectric constant, D1 is the inner diameter of the outer conductor and d is the outer diameter of the inner conductor.

6. A cable as in claim 4 wherein said dielectric means comprises a preformed helix of synthetic resin having a low dielectric constant and a low power factor.

7. A cable as in claim 4 wherein said outer conductor comprises an outer layer of steel and an inner layer of copper electrodeposited thereon, said copper layer being polished.

8. A crush resistant, llexible, high frequency coaxial cable comprising a tubular outer conductor, an inner conductor disposed axially within said outer conductor, and a solid dielectric member extending from said inner conductor to inner face portions of said outer conductor for retaining said conductors in concentric relation throughout the length thereof, said dielectric member comprising a portion of the total volume of space between said conductors, the balance of said space being gas, said outer conductor being transversely c0rrugated, the ratio of the median diameter of said corrugations to the inner diameter of said corrugations being 1.12 whereby to render said cable flexible and to increase the mechanical strength thereof, and whereby to make said cable resistant to crushing forces applied thereto, said outer conductor being operative to maintain said inner conductor in coaxial relation thereto and to maintain optimum electrical characteristics of said cable upon iiexure of said cable over a relatively small radius or upon application of substantial crushing forces to said outer conductor.

9. A crush resistant, ilexible, high frequency coaxial cable comprising a tubular outer conductor, an inner conductor axially disposed within said outer conductor, an annular air space between said conductors, a dielectric member within said space for spacing said conductors, said outer conductor having a longitudinal section of undulating shape wherein all transverse sections taken along the longitudinal extent of said cable have the same area and peripheral outline, said dielectric member extending from said inner conductor to the valley portions of said outer conductor, the ratio of the median diameter of the undulations of said outer conductor to the inner diameter thereof being 1.12 to provide a radial depth for said undulations operative to prevent displacement of the inner conductor relative to the outer conductor and to maintain optimum electrical properties of the cable when said cable is subjected to crushing force or is bent over a small radius.

References Cited in the file of this patent UNITED STATES PATENTS 1,885,168 Aiel Nov. 1, 1932 2,034,047 Leibe Mar. 17, 1936 2,216,435 Eckel Oct. 1, 1940 2,276,084 New Mar. 10, 1942 2,351,056 Lepetit June 13, 1944 2,556,224 Scott June 12, 1951 2,563,578 Candee Aug. 7, 1951 2,576,163 Weston et al Nov. 27, 1951 2,636,083 Phillips Apr. 21, 1953 FOREIGN PATENTS 764,175 France Feb. 26, 1934 140,011 Australia I an. 23, 1951 496,288 Canada Sept. 2, 1953 OTHER REFERENCES Stainless Steel-Sheath Aerial Cable, Electrical Engineering Journal, Ian. 1944, vol. 36, pages 107-109. 

